Field of the Invention
The present invention relates to a method for forming joints between magnesium diboride (MgB2) conductors.
Description of the Prior Art
Magnesium diboride (MgB2) is used as a superconductor for example as filaments in wires used to make superconducting magnets for imaging systems such as MRI systems. The wire must be cryogenically cooled in order to become superconducting.
It has been found difficult to make adequate joints between MgB2 filaments. Either the joints do not become superconducting even at the temperature of operation, or the joints become resistive (known as “quenching”) at an unacceptably low background magnetic field strength or when a current exceeding an unacceptably low threshold is passed.
A typical known process for forming MgB2 joints involves exposing MgB2 filaments from the wires to be joined, pressing them together and exposing them to magnesium powder and boron powder in a mold in a furnace at a temperature in excess of or of order 540° C., under vacuum for outgassing. The powders are mixed and pressed to maximize the density of the resultant MgB2 joint. For example, a weight of 2-30 tons may be used to compress the powders. The compression aims to prevent the resulting joint from being porous, which would reduce the effectiveness of the joint. The temperature is selected to be slightly below the melting point of magnesium or boron, such that the powders do not actually melt, but may be effectively compressed. The elevated temperature ensures that the reaction to produce MgB2 continues at a reasonable rate.
This process results in the deposition of MgB2 on the filaments, providing a joint comprising MgB2 from the filaments of one wire to the filaments of the other wire. In alternative methods, the pressure may be maintained during the heat treatment step, or the pressure may be released once the powders have been compressed, and before the heat treatment step.
In such methods of preparation, the powders are typically made up of particles of approximately 25 μm diameter. During the heat treatment step, the Mg and B react together to form a layer of MgB2, about 2 to 5 μm thick, on the surfaces of the Mg particles. Superconduction between the MgB2 filaments of the joined wires takes place through these surface layers of MgB2. The compression step is required to ensure that the particles are in close contact, to provide an effective conduction path. The heat treatment is carried out at a temperature below the melting point of either Mg or B, but at a high enough temperature that the reaction to create MgB2 occurs at a reasonable rate.
In a typical MgB2 superconducting wire, several MgB2 cores are provided, each sheathed in a protective layer, for example layer of iron, or niobium, or MONEL® alloy. The sheathed cores are then encased in a copper outer to provide mechanical strength and an alternative electrical pathway in case of quench in the MgB2 cores. The sheaths are necessary to prevent the MgB2 cores from reacting with the copper outer, and to provide mechanical strength to the cores during manufacture of the wire. MgB2 is known to be brittle, and will shatter if bent too far.
Conventional jointing processes have included stripping the protective layer from the cores. The exposed cores are then placed in the mold with magnesium and boron powders, as described above, for jointing. Alternatively, the protective layer is not stripped, but the filaments, each comprising an MgB2 core and a protective layer, are cut, or shaved, at a shallow angle such as 2°-5° angle, to expose the core over a relatively large surface area. In an example, the core may be exposed over a length of some 40 mm. These filaments are then placed in a mold with the magnesium and boron powders as described above.
While theoretically attractive, such MgB2 persistent joints have proven to be very difficult to realize. One of the limiting factors is the amount of magnesium oxides found in the deposited MgB2 of the joint. Joints contaminated with magnesium oxides have been found not persistent, that is to say, not superconducting, even with a background magnetic field of 0 T.
It is believed that oxygen outgasses into the vacuum furnace from magnesium oxides present in the magnesium powder used in the process, and possibly also from the structure of the furnace itself. Magnesium oxide MgO dissociates into magnesium and oxygen at a temperature, much below that typically employed to achieve MgB2 formation according to the method described herein.
Conventional methods of joining MgB2 wires comprise the steps of:                exposing at least one MgB2 filament in each of the wires to be joined;        placing the exposed MgB2 filaments in a mold;        adding magnesium and boron powders into the mold;        mechanically pressing the powders in the mold; and        heat treating the filaments and powders to produce an MgB2 joint extending between the filaments of the joined wires.        